In order to connect two workpieces, such as tubular workpieces, to each other to form an angle of, for example, 90°, the workpieces can first be separated in an oblique manner at an angle of 45° and subsequently welded to each other at the cut edges. For the welding operation, the cut edges should abut each other in the most planar manner possible, which may not, however, be possible when the laser cutting beam is orientated perpendicular relative to the surface of the workpiece surface during the cutting operation since, in this instance, warped cut faces can be produced during separation. In order to prevent the warped cut faces, the laser cutting beam and the supersonic flow of cutting gas promoting the laser cutting are inclined at an angle relative to the surface normal, during a process called “oblique laser beam cutting,” in which the angle of inclination is the “oblique cutting angle.” If the oblique cutting angle varies during the cutting operation, even in the case of an oblique cut on a tube, a planar cut face can be produced such that the welding of the cut edges is simplified. The oblique cutting operation can be carried out not only on tubular workpieces, but also on thick, plate-like workpieces in order to enable easier welding of the plate-like workpieces together.
However, the oblique laser beam cutting operation may lead to substantial advance rate reductions (e.g., up to 70% for an oblique cutting angle of 45°) and substantial quality reductions compared to conventional laser beam cutting in which a laser cutting beam is orientated perpendicular with respect to the workpiece surface. The advance rate corresponds to a speed of the relative movement between the laser beam and the workpiece, i.e. the advance rate corresponds to the rate of processing workpieces. For example, the cut edges produced during the oblique laser beam cutting operation can have different surface qualities that vary with the oblique cutting angle. In some cases, a substantial burr may be formed at one cut edge, observable to the naked eye, and a rough surface structure may be formed at the other cut edge.
As explained in “Melt Expulsion by a Coaxial Gas Jet in Trepanning of CMSX-4 with Microsecond Nd:YAG Laser Radiation,” Proceedings of the SPIE, Vol. 5063, trepanning microholes in turbine blades includes orienting the laser cutting beam at an oblique cutting angle with respect to the workpiece, and orienting the supersonic flow of cutting gas or the cutting gas nozzle parallel to the laser cutting beam with a lateral offset in order to position the stagnation point, i.e., the high-pressure region of the supersonic flow of cutting gas directly over the hole. In this manner, periodic variation of the gas pressure and the thickness of the hardened melt along the wall of the hole can be prevented as would otherwise occur if the gas flow and laser beam were oriented coaxially. Due to the lateral displacement of the laser beam and gas flow, the oscillations can be prevented and gas flow through the hole can be increased such that the melt can be more easily discharged at the lower side of the hole. In order to increase the size of a hole obtained by trepanning, an additional hole can placed next to the original hole such that 50% to 80% of the two holes overlap.